The present invention relates to a control system for controlling an implement attached to an agricultural tractor, and, particularly, to a control system that will allow for variations in prevailing slip-pull data at progressively increasing implement draft levels.
The term, xe2x80x9ctractorxe2x80x9d, is meant to include any vehicle capable of propelling a ground or soil engaging implement for the purpose of processing the ground or soil, or objects (e.g. crops, forage, shellfish) lying on or in the ground or soil. Typically a tractor is a four wheel drive vehicle having a hitch for attachment of an implement behind the vehicle. It is also well known for tractors to push implements, such as furrow presses. The typical configuration of a tractor includes an operator cab mounted at the rear of the vehicle, and a forward-mounted engine and transmission system. However it is also known to provide a multi purpose vehicle, that may function as a tractor, having a forward mounted cab and underslung engine and transmission systems beneath a load carrying deck. Other forms of tractors include two wheeled, two wheel drive devices and tracked vehicles that may be coupled to pull or push implements. The instant invention relates to and embraces within its scope all such tractors.
Tractor/implement combinations are widely used in various processes in agriculture. One of the most common of these is plowing, in which a plow is towed behind a tractor. However, tractors may be used for a great variety of other operations such as spraying, furrow pressing, harrowing, raking, seeding and a number of specialized operations such as arise, for example, in vineyards and estuaries, in which specially designed ground-engaging implements are used. Consequently, xe2x80x9cimplementxe2x80x9d as used herein includes but is not limited to plows, harrows, furrow presses, rakes, seed drills, and indeed virtually any article that may be attached to or operated by a tractor and that has the effect of increasing the energy demand of the tractor by virtue of engagement of the implement with the ground or soil or with objects thereon or therein.
Electronic control of the subsystems of tractors is becoming more and more common. For example European Patent Application No. 0838141 (the entire disclosure of which is incorporated herein by reference) discloses an integrated control system for tractors (designated by the trade mark xe2x80x9cTICSxe2x80x9d that is the subject of Community Trade Mark registration no. 1532696), by means of which a programmed microprocessor (or series of microprocessors) maximizes the work rate of a tractor, e.g. during plowing operations, by comparing the implement draft force against a steady state reference model, and performing implement working width and transmission ratio adjustments in order to maintain a maximal work rate while also maintaining a predetermined implement working depth.
There are four readily identifiable subsystems of a tractor/implement combination operating under the control of arrangement such as the aforementioned TICS. The subsystems influence the performance of the combination. They are the tractor engine; the tractor transmission; the implement; and the tire/soil interface. As disclosed in the aforementioned European Patent Application No. 0838141, it has in practice proved impossible successfully to carry out tractor/implement control using a dynamic reference model. The arrangement of the control system in European Patent Application No. 0838141 therefore includes a steady state reference model. In the use of such a model it is necessary for the control software to process accurately generated data on the influence of variables on the behavior and/or performance of the tractor/implement combination.
It is readily possible to obtain real-time data on the engine torque and governor setting, through use of sensors. One suitable form of engine torque sensor is disclosed, for example, in European Patent Application No. 0741286. It is also a straightforward matter to detect, using known transducers, the selected transmission ratio and generate a signal corresponding thereto for use by the control software. Prediction of the horizontally acting load resulting from engagement of the implement with the soil, or with other objects as noted above, is also possible. The method disclosed in European Patent Application No. 0838141 includes for this purpose an assessment of the prevailing soil strength value (or an equivalent thereto in the event of the implement engaging a medium other than soil) during calibration of apparatus included on the tractor/implement combination.
Heretofore, however, there has been no proposal for providing real-time information on the effect of the prevailing tire/soil interface on the performance of a tractor/implement combination. When a wheeled tractor propels an implement either by towing it or by pushing it, a degree of so-called xe2x80x9cwheel slipxe2x80x9d arises. Wheel slip, that is expressed as a percentage value, varies in dependence on numerous factors including the traction factors or conditions, that in turn depend on the soil type and density, the soil moisture conditions, and the presence at the soil surface of e.g. crop residues; and vehicle factors, including the tire size, the tire condition, the ballasting (weight distribution) of the tractor, and whether the tractor is a two wheel drive (2WD) or four wheel drive (4WD) vehicle.
The tire size and condition (that determine the area of the tire surface in contact with the soil) do not in practice vary during e.g. a plowing operation. Similarly the vehicle ballasting is, in Northern Europe at least, likely to be invariant during e.g. a plowing operation. This is because in Northern Europe the only factor that is likely to cause variations in the vehicle ballasting is the gradual depletion of fuel in the tanks of the tractor. This mass change is insignificant compared with the mass of the tractor. In North America it is known to inject nitrous ammonia during soil tilling operations. The nitrous ammonia is typically stored in a tank at the front of the tractor. Reduction of the level of nitrous ammonia during tilling may (depending on the mass of nitrous ammonia dispensed) have a noticeable effect on the ballasting of the tractor.
For a given tractor and implement combination it is possible to derive a so-called xe2x80x9cslip-pullxe2x80x9d curve that is a plot of the percentage wheel slip (y-axis) against the horizontally acting load resulting (referred to as the draft, in kN, when the tractor tows an implement) from engagement of the implement (x-axis). The term xe2x80x9cslip-pullxe2x80x9d is used even when the tractor is arranged to push rather than tow an implement.
In the past the possibility of variations in slip-pull characteristics have been largely ignored in the software responsible for predicting the performance of a tractor/implement combination. Instead it has been the practice simply to employ a one-dimensional lookup table, stored in a memory forming part of the control apparatus, that represents an idealized slip-pull curve of use of a tractor/implement combination in a xe2x80x9csandy loam stubblexe2x80x9d (SLS) soil. This approach was generally acceptable for the following reasons:
(a) The SLS slip-pull curve is fairly conservative. Pull that can be generated at a given slip level (for the same tire size and vehicle mass) is dependent on inherent soil strength and the frictional nature of the surface. Hence while a sandy soil will return similar slip-pull characteristics over a wide range of moisture contents (until its bearing capacity is eventually reduced); a clay-based soil is inherently stronger and can therefore generate greater traction. However the range of moisture contents over which this can be achieved is narrower, increasing moisture causing a rapid increase in wheel slip. Consequently in the majority of field conditions in which plowing would be contemplated, the SLS curve returns an acceptable estimate (or possible underestimate) of the pull levels that can be generated at any given slip.
(b) Additionally a tire-soil traction system is a relatively stable, forgiving system at the slip levels which TICS tries to operate for maximum field efficiency (11-14% slip). At times (in dry, good traction conditions), the traction interface may have appeared under-loaded (8-10% slip), but this is often a result of a compromise between available engine power, implement-imposed draft and vehicle ballasting and tire size.
Despite the generally acceptability of the single SLS slip-pull curve as discussed, a need has arisen for greater robustness of the control. This need has arisen principally from use of tractor/implement combinations in soils that do not closely match the SLS soil on which the stored SLS slip-pull curve is based; and use of a mis-matched tractor and implement combination.
It is, therefore, an object of the present invention to provide a method of controlling an implement attached to a tractor that solves the aforementioned problems of prior art control systems.
It is a feature of this invention that the method of controlling a tractor/implement combination can be utilized with an implement that is not adjustable.
It is an advantage of this invention that the method of controlling the combination of a tractor and attached implement advantageously improves the robustness and accuracy of control of the tractor/implement combination.
It is another object of this invention to provide a method of controlling the combination of a tractor and attached implement that is suitable for use with a tractor/implement combination in which one or more features of the implement are adjustable so as to vary the horizontally acting loading (pull) experienced by the tractor.
It is another feature of this invention that the method includes selection of a transmission ratio that is known to be suitable for the tractor/implement combination and for the task under consideration.
It is still another feature of this invention that an experienced tractor operator will know that in carrying out some tasks only a limited range of the tractor transmission ratios is suitable, so that the method advantageously embraces within its scope selecting the transmission ratio selected from the group of suitable ratios for the task in question.
It is another advantage of this invention that the predetermined engine governor setting will be used as the setting corresponding to maximum speed of the tractor engine in the selected transmission ratio.
It is another advantage of this invention that the method of controlling the tractor/implement combination provides a convenient datum setting, although other governor settings may if desired be selected.
It is still another feature of this invention that the step of adjusting one or more settings of the implement, when carried out, includes increasing the working depth of a depth-adjustable implement.
It is still another advantage of this invention that the step of adjusting implement settings, when carried out repeatedly, sequentially increases the draft experienced at the tractor/implement hitch, thereby permitting the calibration method to be carried out over a range of loadings.
It is yet another feature of this invention that the step of adjusting one or more settings of the implement may include increasing the width of a width-adjustable implement.
It is yet another advantage of this invention that the step of increasing the width of a width adjustable implement, when carried out repeatedly, also sequentially increases the loading of the tractor.
It is still another feature of this invention to adjust the implement depth to a predetermined value before adjustment of the element width of the implement commences.
This preferred order of the method steps is advantageous because under some soil conditions and with some tractor/implement combinations, it is possible, through adjustment of the implement depth alone, to achieve a degree of wheel slip that exceeds the normal working range. Under such circumstances there is no need additionally to increase the width of the implement in order to provide a full range of test loadings for the tractor/implement combination. This is preferable because tractor operators generally prefer to maintain a constant furrow width when plowing a field.
Nonetheless, within the scope of the invention it is possible to permit further adjustment of the implement depth after commencement of the adjustment of the implement width. This possibility allows numerous options for increasing the tractor wheel slip to a predetermined threshold value.
Preferably the predetermined wheel slip threshold value is 28%. Once the horizontally acting load on the tractor is sufficient, through practicing of the method of the invention, to cause this degree of wheel slip the tractor is operating well outside its normal, efficient range. Currently therefore there is therefore no requirement to obtain further data after a 28% wheel slip value has been reached.
It is still another object of this invention to provide a method of controlling a tractor/implement combination that includes the step of comparing the stored values with reference data includes the sub steps of:
(a) determining from the stored values a reference value of the horizontally acting load corresponding to a predetermined reference wheel slip value; and
(b) mapping the reference value of the horizontally acting load onto a series of reference data to enable selection of a set of the reference data.
More specifically the substep of determining the stored values includes the further substep of:
(c) determining the value of the horizontally acting load at the reference wheel slip value, by interpolation between two or more said values that are stored in a memory device.
It is yet another object of this invention to provide a method of controlling the combination of a tractor and attached implement that incorporates reference data including a plurality of characteristic wheel slip/horizontally acting load (slip-pull) curves; and the step of selecting a set of reference data includes the sub-step of:
(a) selecting a said curve by identifying the curve, from the plurality, that approximates most accurately to the reference value of the horizontally acting load at the reference wheel slip value.
These features advantageously allow for straightforward manipulation of the recorded data and their comparison with pre-existing slip-pull curves. The use of a plurality of slip-pull curves greatly increases the accuracy of the model, compared with the prior art arrangement that used only a single such curve.
In all probability the reference value of the horizontally acting load at the reference wheel slip value, that in a preferred embodiment is 25% wheel slip, does not lie exactly on one of the slip-pull curves. Therefore, it is a further object of this invention to provide a method of controlling a tractor/implement combination that includes the addition of positive and negative tolerances to respective values represented by the reference curves at the said reference wheel slip value. In other words, the method includes effectively xe2x80x9cbroadeningxe2x80x9d the curves at least at the reference wheel slip value, so that any given reference horizontally acting load value at the reference wheel slip value will intersect one of the curves.
To ensure robustness of this technique, the modulus of the positive tolerance added to each said reference value is greater than the modulus of the negative tolerance added thereto. For this reason it is also desirable that the sum of the modulus of the said positive tolerance added to a first said referenced curve and the modulus of the said negative tolerance added to the next successive reference curve along a line representing the said reference wheel slip value is equal to the distance along the said line by which the said first and second reference curves are separated one from another.
The foregoing features ensure that the xe2x80x9cbroadeningxe2x80x9d of the reference slip-pull curve is sufficient that any reference horizontally acting load value likely to be recorded during practicing of the method will intersect one of the curves.
It is yet another feature of this invention that the ratio of the modulus of the positive tolerance to the modulus of the negative tolerance is 3:2.
It is still another feature of this invention that the reference curves are stored as a two-dimensional lookup table in a memory.
It is a further feature of this invention that the method of controlling a tractor/implement combination can include the following optional steps:
(a) detecting whether, during step (i), the tractor engine speed is less than a predetermined minimum; and
(b) detecting whether, during step (i), the tractor wheel slip exceeds a predetermined initial wheel slip maximum.
In either case it is possible, as a result of such detection, as necessary to initiate a further control action. For example it is possible for the method to include the transmission of a message to a cab-mounted display device, to the effect that an engine stall is imminent by virtue of the calibration run being attempted in too high a transmission ratio or for a similar reason; or a message indicating that the draft loading caused by the implement is causing too high a degree of wheel slip for the calibration meaningfully to be carried out.
According to a second aspect of the invention there is provided a method of controlling a tractor/implement combination including the steps of:
(a) carrying out a method as defined herein; and
(b) carrying out a control action using the resulting selected set of reference data.
The control action may, for example, include operation of a software program of the kind described in European Patent Application No. 0838141, using the resulting, selected set of reference data as an input thereto.
These and other objects, features and advantages are accomplished according to the instant invention by providing a method of controlling the combination of a tractor and an attached implement includes the calibration of a tractor/implement combination to allow for variations in prevailing slip-pull data at progressively increasing implement draft levels. The recorded data is then interpolated at a reference slip value and compared with a series of reference slip-pull curves. The slip-pull curve approximately most closely to the recorded pull value at the reference slip value is then selected for subsequent use in a control algorithm.